1. Field of the Invention
The present invention relates to an array antenna system and in particular to a technique of calculating antenna weights for null direction control.
2. Description of the Prior Art
In base stations of a mobile communications system, signals received by respective antenna elements of an array antenna are subjected to adaptive signal processing to form nulls in incoming directions of interference waves, which allows the interference to be suppressed. In addition, the null pattern obtained from the received signals is also used for signal transmission.
In the case of asymmetric communication such as Web access using ADSL (asymmetric digital subscriber line) service, however, the null pattern obtained from the received signals is not always best suited for transmission. In this case, it is necessary to determine null directions in some way and form nulls in the determined directions.
Antenna weights forming nulls in desired directions can be obtained by using a Howells-Applebaum adaptive array control algorithm in a model which is formed when the antenna weights are calculated and receives a signal wave and interference waves at designated directions. Details of the Howells-Applebaum adaptive array control algorithm are discussed in, for example, Chapter 4 titled MSN adaptive array, pp. 67-86, xe2x80x9cAdaptive Signal Processing by Array Antennaxe2x80x9d by Nobuo Kikuma, SciTech Press.
FIG 1 is a flow chart showing a conventional null direction control method using the Howells-Applebaum adaptive array control algorithm. When null and beam forming directions, xcex8 beam, xcex8 null(l), . . . , xcex8 null(M), are designated, steering vectors, Abeam, Anull_1, . . . , Anull_M, in the null and bean forming directions are generated and then are combined to produce Asum. The combined steering vectors Asum is used to calbulate a covariance matrix RAA. An inverse matrix of RAA is used to calculate the optimum weights, Wbeam, of the array antenna.
However, the optimum weight computation according to the above prior art needs the inverse matrix calculation. This causes processing time and amount of calculation to be increased, resulting in lowered processing speed and increased amount of hardware.
An object of the present invention is to provide a null direction control method which can obtain optimum antenna weights forming designated null beam directions without calculating an inverse matrix.
In an N-element array antenna, a designated null beam antenna pattern is obtained by processing a 2-element antenna weight vector forming a null in a sequentially selected one of M designated null directions and a (Nxe2x88x92M)-element antenna weight vector forming a beam in a designated beam direction to produce an antenna weight vector for the N-element array antenna. The final antenna weight vector is calculated by incrementing the number of elements of a work antenna weight vector each time a null is formed in a sequentially selected one of the M designated null directions.
According to an aspect of the present invention, a method for producing an antenna weight vector for an N-element array antenna to form a designated antenna pattern having a single beam direction xcex8 beam and M null directions xcex8 null(1)-xcex8 null (M) (1= less than M= less than Nxe2x88x922), includes the steps of: a) producing a work antenna weight vector for a (Nxe2x88x92M)-element array antenna to form a beam in the single beam direction; b) sequentially selecting one of the M null directions; c) producing a 2-element antenna weight vector for a 2-element array antenna to form a null in the selected null direction; d) multiplying the work antenna weight vector by a first weight and a second weight of the 2-element antenna weight vector to produce a first work weight vector and a second work antenna weight vector; e) appending 0 to a trail end of the first work weight vector and to a head of the second work weight vector to produce a first expanded weight vector and a second expanded weight vector, and adding the first expanded weight vector and the second expanded weight vector to produce a work antenna weight vector; and f) repeating the steps (c)-(e) until antenna weight vector as the antenna weight vector for an N-element array antenna.
The step (a) may include the step of calculating the work antenna weight vector Wpattern=[Wbeam(1), . . . , Wbeam(Nxe2x88x92M)] using the following expressions:
xcex4wbeam=exp{xe2x88x92jxc2x7kxc2x7dxc2x7sin(xcex8 beam)},
wbeam(l)=1,
and
wbeam(i)=wbeam(ixe2x88x921)xc2x7xcex4wbeam (i=2, 3, . . . , Nxe2x88x92M),
where d is a distance between antenna elements of the N-element array antenna, k is propagation constant of free space (k=2xcfx80/xcex), xcex is wavelength in free space.
The step (c) may include the step of calculating the 2-element antenna weight vector Wnull(m)=[wnullxe2x80x941(m), wnullxe2x80x942(m)] using the following expressions:
xcex4wnull(m)=xe2x88x92exp{xe2x88x92jxc2x7kxc2x7dxc2x7sin(xcex8 null(m)) },
wnullxe2x80x941(m)=1,
and
                              w                      null_            ⁢            2            ⁢                          (              m              )                                      =                              w                          null_              ⁢              1              ⁢                              (                m                )                                              ·                      δw                          null              ⁡                              (                m                )                                                                                      =                                    -              exp                        ⁢                          {                                                -                  j                                ·                k                ·                d                ·                                  sin                  ⁡                                      (                                          θ                      ⁢                                              xe2x80x83                                            ⁢                                              null                        ⁡                                                  (                          m                          )                                                                                      )                                                              }                                      ,            
where m=1, 2, . . . , M.
The step (d) may include the step of calculating the first work weight vector Wbeam1 and the second work antenna weight vector Wbeam2 using the following expressions:
Wbeam1=wnullxe2x80x941(m)xc2x7Wpattern=1xc2x7Wpattern,
and
                              w          beam2                =                              w                          null_              ⁢              2              ⁢                              (                m                )                                              ·                      w            pattern                                                  =                  exp          ⁢                                    {                                                -                  j                                ·                k                ·                d                ·                                  cos                  ⁡                                      (                                          θ                      ⁢                                              xe2x80x83                                            ⁢                                              null                        ⁡                                                  (                          m                          )                                                                                      )                                                              }                        ·                                          w                pattern                            .                                          
The step (e) may include the steps of: appending 0 to the trail end of the first work weight vector Wbeam1 and to the head of the second work weight vector Wbeam2 to produce the first expanded weight vector [Wbeam1, 0] and the second expanded weight vector [0, Wbeam2]; and adding the first expanded weight vector and the second expanded weight vector to produce the work antenna weight vector
Wpattern=[Wbeam1, 0]+[0, Wbeam2].
According to anther aspect of the present invention, a method for producing an antenna weight vector for an N-element array antenna to form a designated antenna pattern having M null directions xcex8 null(1)-xcex8 null(M) (1= less than M= less than Nxe2x88x921), includes the steps of: a) arbitrarily preparing a work antenna weight vector for a (Nxe2x88x92M)-element array antenna; b) sequentially selecting one of the M null directions; c) producing a 2-element antenna weight vector for a 2-element array antenna to form a null in the selected null direction; d) multiplying the work antenna weight vector by a first weight and a second weight of the 2-element antenna weight vector to produce a first work weight vector and a second work antenna weight vector; e) appending 0 to a trail end of the first work weight vector and to a head of the second work weight vector to produce a first expanded weight vector and a second expanded weight vector, and adding the first expanded weight vector and the second expanded weight vector to produce a work antenna weight vector; and f) repeating the steps (c)-(e) until the M null directions have been selected, to produce a fluid work antenna weight vector as the antenna weight vector for an N-element array antenna.